Prawn farming in Senegal may hold the key to eradicating a common and deadly parasitic disease.

Researchers believe if the shell fish are reintroduced into the West African nation's rivers, they will eat the snails that host the parasite that causes schistosomiasis.

Spread through contaminated water, the disease, also known as bilharzia, kills more than 200,000 people a year, according to UN figures.

More than two million more are infected each year by the parasitic worms which impair child growth and damage internal organs.

My father used to fish hundreds of prawns every day. Now it's very difficult for us, the younger generation, to make a living out of fishingFisherman Batch Boye

"I've been infected with schistosomiasis for two years," says Gadiaga Diop from the village of Lampsar, about 20km (12 miles) from the city of Saint Louis.

Nearby children play and bathe in the sun as mothers wash household dishes and clothes in the Senegal River.

It is a beguiling scene - as the water is dangerous with a 60% prevalence rate of schistosomiasis.

"It started with painful and bloody urination," says Ms Diop.

"A doctor gave me pills, but they had side effects, so I also vomited, and had diarrhoea.

"I was very tired, I lost weight, and I was afraid for my life."

Schistosomiasis is the second most common parasitic disease in the world after malaria - with 90% of cases in Africa.

Complications include profuse bleeding in the digestive system that can lead to death.

The infection can be treated fairly effectively with a drug called praziquantel.

No running water

The latest government campaign to distribute the pill has lowered bilharzia cases around Lampsar village from about 30 a month to less than 10, says Fatou Sarr Diouf, head of the regional health centre.

But there is nothing to prevent re-infection.

The government has put up posters and organised talks to explain to the villagers that urinating in the water can make them sick, and that they should avoid bathing in the river at noon - the snails come out when the temperature is highest.

But for as long as people are exposed to the river, they will be exposed to the disease.

Image caption
At market giant river prawns sell for five times the price of fish

"I know it's because I keep going back to the river, but there is no running water here and I have to go there at least twice a day, to wash the dishes, do my laundry, wash my children. I'm afraid every time I go in, but I have no choice."

Scientists have been looking for ways to definitively eliminate the disease for years, especially since the 1980s, when there was an unprecedented bilharzia outbreak shortly after a dam was built on the Senegal River.

An organisation called Espoir Pour La Sante has been working on a vaccine for 20 years - and the results of the latest set of tests are due this month.

Those behind a new non-profit scheme called Project Crevette (Prawn) hope that by reintroducing prawns into the Senegal River, not only will the causes of the disease be wiped out, but the region will also benefit economically.

The idea was born after scientist Armand Kuris, from the University of California, proved that prawns eat the mollusc hosts. He shared his findings with Elizabeth Huttinger, who worked on public health development projects and went on to found Project Crevette.

"I realised immediately that the idea of raising prawns and selling them through micro-commerce meant that the health effect could be sustainable," she says.

Fewer snails

Project Crevette is still at an experimental phase.

Schistosomiasis

Also known as bilharzia

Affects more than 230 million people worldwide a year

The schistosomiasis-causing parasite is released by freshwater snails

The parasites are released into the water, and use fork tails to burrow into the skin

They travel to blood vessels that supply urinary and intestinal organs, including the liver, where they mature

Female worms, which live inside the thicker males, release many thousands of eggs each day

Eggs shed in urine and faeces may make their way into snail-inhabited water, where they hatch to release parasites that seek out snails to begin the cycle again

Researchers first set up an enclosure around an area at one end of Lampsar village popular with bathers and filled it with more than 100 prawns.

Another bathing site on the other side of the village was left untouched.

Then 300 people were treated for schistosomiasis from the two sections of the village.

Six months later, they were tested and it was found that there was a lower infection rate amongst those who bathed near the prawns - 80% lower than the area where there were no prawns.

The researchers also reported that there were fewer snails in the areas of the river where the prawns were introduced.

Before the dam was built, prawns were an important source of revenue for the locals.

"My father used to fish hundreds of prawns every day," recalls Batch Boye, a fisherman who works near the dam.

Image caption
Bilharzia infection rates were lower near the prawn enclosure on the river

"They sold well. Now it's very difficult for us, the younger generation, to make a living out of fishing."

But if the Project Crevette experiment continues to be conclusive, the biggest challenge will be to find a way to sustainably restore prawns to the river.

The river used to be their natural habitat, but the dam prevented the prawns from accessing salty water where they reproduce.

The prawns being used in the experiments were imported from Cameroon, but Project Crevette wants to breed its prawns locally in the future and to involve local communities as much as possible.

A few months ago it set up a hatchery at the National Aquaculture Agency and is training four water farming students from Saint Louis University to breed prawns.

They are hoping profit may prove a motivator.

Ahmadou Tidjane Camara, head of the National Aquaculture Agency, who backs the reintroduction, says the project is an opportunity for local technicians to learn to breed prawns.

"The prawns could be a great source of profit for poor populations in the area," he says.